This invention relates generally to the field of absorption chillers, and more particularly to a non-linear controller for an absorption chiller.
In an absorption chiller, the chilled water temperature in the leaving chilled water line is directly affected by disturbances such as the entering chilled water temperature and the entering cooling water temperature. Because the only control point for the system is a capacity valve which controls the heat to the system, whether from steam or gas flame, and because the system is chemical-based, the machine dynamics of the system are relatively slow. Changes created by the disturbances mentioned above are removed slowly by the existing capacity control.
Briefly stated, in an absorption chiller system, a control input for the chiller is a heat source controlled by a capacity valve, which is in turn controlled by a PI controller. The controller is controlled by a non-linear control function. During operation, a disturbance in the system is measured. A signal error is defined as a setpoint for the leaving chilled water minus the disturbance. The non-linear control function is represented as C(s)=KP0(1+b|E|)+KI/s, where where KP0 is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and KI is an integral gain.
According to an embodiment of the invention, a method for controlling an absorption chiller system, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a PI controller, includes the steps of (a) measuring a disturbance in said system; (b) defining a signal error as a setpoint minus said disturbance; and (c) controlling said capacity valve based on a control function in said PI controller, wherein said control function is represented by C(s)=KP0(1+b|E|)+KI/s, where where KP0 is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and KI is an integral gain.
According to an embodiment of the invention, a control system for an absorption chiller, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a PI controller, includes means for measuring a disturbance in said chiller; means for defining a signal error as a setpoint minus said disturbance; and means for controlling said capacity valve based on a control function in said PI controller, wherein said control function is represented by C(s)=KP0(1+b|E|)+KI/s, where where KP0 is the gain when said signal error is zero, |E| is the absolute value of the signal error, b is an adjustable constant, and KI is an integral gain.